How did Islam spread among other peoples if the Prophet was illiterate and the Qur'an was revealed in Arabic, and was the Prophet's message limited to Arabs or did it include other peoples?

The fact that the Messenger was illiterate is established in the Quran and Sunnah, as in His (Allah's) saying: ﴿So believe in Allah and His Messenger, the unlettered prophet﴾, and his (the Prophet's) saying: "We are an unlettered nation; we do not write or calculate." His message is universal for all people, Arabs and non-Arabs alike, as stated by Allah Almighty: ﴿And We have not sent you except comprehensively to all people as a bringer of good tidings and a warner﴾, and his (the Prophet's) saying: "A prophet used to be sent specifically to his people, but I have been sent to all people." He had scribes who wrote his letters to kings, such as his letter to Heraclius, and he commanded some of his companions to learn Syriac.